There are a number of compact vehicles that need to have “automatic” transmissions for ease of driving and for increased utility. These include ATV's, tractors, utility work vehicles and small automobiles. These vehicles are generally in the 25 HP to 50 HP range, and have common requirements for low cost, high efficiency, good controllability and continuous ratio change throughout the entire speed range. These vehicles are small and need transmission packages which are short and narrow and which have inputs and outputs conveniently located.
There can be a wide range in the required transmission ratio spread that varies by vehicle vocation. Further, the transmission configuration varies with the specific vehicle design. Both of these issues can be major determinants of cost. There are some differences in engine speed, which can affect the sizing of the transmission components.
Therefore, a principal object of this invention is to provide a hydrostatic mechanical transmission (HMT) which accommodates the range of vehicle heeds with a basic design approach, and provides for the adapting of different vehicle requirements while retaining many key transmission components across a range of vehicles.
Further objects of the invention are as follows:
1. To provide an HMT with a continuous ratio from full reverse to full forward speed in a compact vehicle. Providing controlled output speed through zero eliminates the need for any clutch between the engine and transmission.
2. To provide controlled output speed which can be configured in either a 2-mode or 3-mode version depending in the application requirement. The third mode is independent in ratio spread from the other two modes.
3. To provide a transmission configuration which has a center housing portion which contains features and location for two hydrostatic units which is common across the range of transmission applications, and two end covers for the center portion which contain the features and location for the mechanical shafts, engine mounting, and PTO drive. The housing split lines are located on the front and rear of the V and F hydrostatic assemblies.
4. To provide for the transmission output shaft location to be below and offset to one side of the input shaft, so as to allow for routing of the driveshaft(s) close to the engine, in either an integrated or non-integrated engine/transmission configuration.
Vehicle Background:
The vehicles intended for application of this transmission have a single seat for the driver who typically sits close to the engine/transmission package and may straddle it. The transmission must be compact and allow routing of the driveshaft below/beside the engine. It is desirable to have a continuous ratio throughout the vehicle speed range in order to allow maximum flexibility for the driver or work to be done. Minimum cost is achieved with no gears between the front and rear driveshafts and with no clutch between the engine and transmission.
Transmission Background:
Hydromechanical transmissions are characterized by a hydrostatic transmission power path in parallel with a mechanical power transmission path, arranged in a manner to decrease the average power flow through the hydrostatic portion and thereby increase operating efficiency. Typically, the mechanical power path includes a planetary gear set which acts to sum the power flows at either the input or output end of the transmission.
The existence of parallel power paths creates the possibility of reducing the output speed range or torque ratio in order to further reduce transmitted hydrostatic power; this then requires multiple ranges or “modes” to achieve the full torque and speed range of the transmission. The impact of multiple modes is to improve efficiency and sometimes to reduce cost. In addition to efficiency and cost, the magnitude of the output speed range/torque ratio in each mode has an impact on input power capacity relative to the size of the HST. Smaller ratios allow larger input power for the same size hydrostatic units. It is obvious that more modes allow either smaller mode ratios or larger transmission ratios or both. These relationships create the possibility for having a versatile design configuration that accommodates a number of market needs for input power, ratio range and efficiency.
Since a hydrostatic transmission is a part of the unit, one or more of the modes can be hydrostatic, or without parallel power paths. If there is a hydrostatic mode, it is usually the start-up range, or mode 1.
Multi-mode HMT's are usually accomplished by reusing the hydrostatic components and clutching to a different mechanical component. The mechanical component will be a planetary if the mode is hydromechanical. Usually the modes are arranged so that there is no ratio change during the mode change in order to have continuous speed or torque delivery. Also, the hydrostatic transmission is usually stroked over center from full positive displacement to full negative displacement in order to fully utilize the installed hydrostatic power. When making a mode change, a planetary element different from any other mode must be used if the speed/torque ratio of the mode is to be independently selected from the other modes.